A longstanding problem in the production of polyolefins in fluidized bed reactors is the loss of the unreacted monomer in the fluidizing gas as a consequence of removing the solid polymer product from the reactor. The particles of polymer or resin product are generally removed by passing them into a product tank connected with the fluidized bed reactor, which usually occurs under positive gas pressure assisted by gravity, during which the particles are still somewhat entrained in or surrounded by the gaseous atmosphere of the reactor. Regardless of the system of valves through which solid product flows during a discharge cycle, the cycle gas leaving the reactor along with the product generally contains a significant volume of unreacted monomer. The loss of unreacted monomer constitutes an economic loss of the chemical reactant. Fluidizing gas may further include inert gases, for example, to control the dew point of the recycle gas and which also represents an economic loss each time the polymer product is removed from the reactor.
Previous systems that have been developed to minimize the loss of gas when the polymer product is removed tend to impede the removal of the solid product itself, relative to a reactor design not employing such a system. As a result, the entire polymerization process ultimately can be limited by the rate of product removal, rather than the depend on the rate of production. These production limitations in large, expensive reactors are highly undesirable.
Therefore, there exists a need for new product removal strategies that can help minimize the loss of reactor gases when the polymer product is removed, but which do not hinder the removal of the solid product during product discharge. There is also a need for product removal systems that can operate over a wide range of reactor parameters, without limiting the rate of production.